MINOR: otel: test: unified run scripts into a single symlinked script

Replaced SH_ARGS variables with 'set --' and "${@}" to ensure proper
quoting of haproxy command-line arguments.  Then replaced individual
per-config run scripts with a single generic run-test-config.sh that
derives the configuration directory from its own filename.  The former
scripts became symlinks, and a new run-empty.sh symlink was added.

addons/otel/README-configuration

@@ -568,6 +568,13 @@ Supported types:
     http_headers                    custom HTTP headers (list of key: value)
     max_concurrent_requests         concurrent request limit (integer)
     max_requests_per_connection     request limit per connection (integer)
+    background_thread_wait_for      idle timeout for the HTTP background thread
+                                    in milliseconds; 0 means the thread never
+                                    exits on its own (integer, default: 0).  If
+                                    this option is set, 'insecure-fork-wanted'
+                                    must be used in the HAProxy configuration,
+                                    otherwise HAProxy may crash while exporting
+                                    OTel data
     ssl_insecure_skip_verify        skip TLS certificate verification (boolean)
     ssl_ca_cert_path                CA certificate file path (string)
     ssl_ca_cert_string              CA certificate as inline string (string)

addons/otel/test/sa/otel.yml

@@ -29,6 +29,7 @@ exporters:
   exporter_traces_otlp_http:
     type:                        otlp_http
     thread_name:                 "OTLP/HTTP trace"
+#   background_thread_wait_for:  10000
     endpoint:                    "http://localhost:4318/v1/traces"
     content_type:                json
     json_bytes_mapping:          hexid
@@ -91,6 +92,7 @@ exporters:
   exporter_metrics_otlp_http:
     type:                        otlp_http
     thread_name:                 "OTLP/HTTP metr"
+#   background_thread_wait_for:  15000
     endpoint:                    "http://localhost:4318/v1/metrics"
     content_type:                json
     debug:                       false
@@ -133,6 +135,7 @@ exporters:
   exporter_logs_otlp_http:
     type:                        otlp_http
     thread_name:                 "OTLP/HTTP logs"
+#   background_thread_wait_for:  20000
     endpoint:                    "http://localhost:4318/v1/logs"
     content_type:                json
     debug:                       false